1. Field of the Invention
The present invention relates to a solid-state image pick-up apparatus and a drive method therefor, and more particularly to such an apparatus and a method for producing an image signal with dark current minimized.
2. Description of the Background Art
An image capturing apparatus having a solid-state image pick-up device is required to produce an image signal whose quality is as high as possible on its output. When influence of a noise component due to dark current included in an image signal increases, signal to noise ratio is deteriorated. In order to address this problem, some methods as disclosed by U.S. Pat. No. 5,900,769 to Tanabe and Japanese Patent Laid-Open Publication No. 279608/1996 have been proposed. Tanabe and Japanese '608 publication both disclose a method for reducing dark current of an FIT (Frame Interline Transfer) type of charge-coupled device (CCD).
More specifically, Tanabe discloses technology which has signal charges wait under the effect of a surface potential similar to the pinning effect of photosensitive cells in a horizontal scanning period so that no dark current is produced. Further, Japanese '608 publication discloses technology in which a transfer path is composed of a two-phase clock driven type of CCD device having its vertical transfer electrode applied with a negative voltage during a horizontal scanning period so that the surface potential of the well is “pinned” to the storage level of the charge packet.
Note that dark current is primarily generated on vertical transfer paths composed of CCD devices. In accordance with the technology disclosed by Tanabe and Japanese '608 publication, the electric charge storage portions, such as vertical transfer paths, in which signal charges stay for a longer period of time, should be pinned rather than others. However, since the vertical transfer paths are of an IT (Interline Transfer) type of CCD devices, which are different in CCD structure from the photosensitive cells, the methods for reduction of dark current disclosed by both publications are not applicable to the vertical transfer paths.
The interline transfer type of CCD is different from the storage portions which are of a frame interline transfer type of CCD. Typically, the IT type of CCD has electrodes connected to transfer shift gates for reading out signal charges accumulated in photosensitive cells to a vertical transfer path, i.e., electrodes having transfer gates connected, and electrodes without transfer gates, and both types of electrodes are alternately arranged, and are driven in a four-phase or eight-phase driving fashion, for example.
Another Japanese Patent Laid-Open Publication No. 2005-286470 discloses a method for reducing dark current by reducing the capacity of potential wells formed not only in FIT type of CCDs but also vertical transfer paths. Reduction of the capacity of potential wells is usually associated with the reduction of the area of the potential wells formed in the vertical transfer paths.
The studies to detect the predominant causes of dark current generated in the vertical transfer paths was conducted, and show that dark current increases when signal charges stay longer below the electrodes having the transfer gates connected. This implies that the method, disclosed in Japanese '470 publication, for reducing the capacity of potential wells formed in vertical transfer paths does not provide a way to effectively reduce dark current.